1. Field of the Invention
This invention relates to an improved process for the extrusion of difficultly-melt-processible polymers.
2. Background
In the extrusion of difficultly-melt-processible polymer resins there are often flow regimes, determined by the rheological properties of the particular resin, where anomalous flow behavior occurs, leading to imperfections on the extrudate surfaces. Such imperfections, commonly called melt fracture, appear in different forms. The so-called "sharkskin" fracture occurs at lower shear rates and appears as a general, finely-structured and uniform roughness. In a blown-film extrusion, sharkskin fracture may appear as an undesirable herringbone pattern, reducing clarity and giving a dull surface. In practice this may occur at uneconomically low extrusion rates. At higher shear rates flow often becomes unstable and a non-uniform stick-slip melt fracture results, wherein alternating bands of glossy surface and sharkskin fracture appear. This behavior is especially undesirable in wire coating and in tube and pipe extrusions. Other recognized difficulties that occur during polymer extrusion include fluctuations in barrel and die pressure, torquing out because of excessively high pressures reached during fluctuation, and accumulation of degraded polymer and polymer additives at the die exit orifice.
In an effort to improve the extrusion behavior of polymer resins through metal dies it is known to coat the die surfaces that contact the flowing polymer melt with a slip agent, such as tetrafluoroethylene polymers and copolymers, as in Japanese Application Publication Kokai 55-82784, but bonding to the metal is poor, and over a period of time in use the slip layer is depleted and melt fracture resumes.
It is also known, as disclosed by Blatz in U.S. Pat. No. 3,125,547, that incorporation of 0.005-2.0 wt. % of an incompatible fluorocarbon polymer that is above its glass transition temperature if amorphous, or above its melting point, if crystalline, at the processing temperature, such as a fluoroelastomer or polyvinylidene fluoride, into linear low density polyethylene and other polyolefins will reduce die pressure and significantly increase the extrusion rate at which melt fracture occurs. U.S. Pat. No. 4,904,735 discloses the use of certain combinations of fluoropolymer process aids for achieving similar results.
In the practice of the teachings of the art, wherein a fluoropolymer process aid is added, the fluoropolymer contacts metal surfaces of the extruder and die parts and preferentially collects there, thus providing a low surface energy slip layer between the extruding resin melt and the metal surfaces. In commercial extrusions it is often a practice to add up to 1 wt. % of a finely divided talc or silica to the polyolefin as an antiblock agent. This practice can greatly diminish the beneficial effects of the fluorocarbon polymer additive as the antiblock agent may remove, for example, by abrasion, or displace the lubricating layer of fluorocarbon polymer that is relatively weakly adhered to the die surface, especially if the die is not clean. Moreover, purge compounds used to clean extruders and dies often contain chemical additives, such as stearates, and abrasives that collect on the die and seriously interfere with the adhesion and performance of fluoropolymer process aids in subsequent extrusions.
It is known to use as process aid additives in difficultly-melt-processible resins certain fluorinated polymers having --SO3M groups (wherein M is a metal ion), said fluorinated polymers being used either alone, as in Japanese Examined Publication Kokoku 55544/1988, or in combination with a perfluoroalkyl sulfonate salt, as in Japanese Examined Publication Kokoku 55543/1988. These additives are very expensive, and must be continuously present in the extruding resin.
It is an object of this invention to provide a die treatment process whereby the extrusion of difficultly-melt processible polymers containing fluorocarbon polymer process aids is greatly improved, especially in the presence of abrasive additives, such as antiblock agents.